1. Field of the Invention
The present invention relates to a rewritable optical information recording medium including two substrates assembled together and a method for producing the same.
2. Description of the Related Art
Technologies for performing high-density recording of information and reproduction of such information using laser beams are known. For example, optical disks have been put into practical use. Optical disks are roughly classified into read-only type, write-once type, and rewritable type. The read-only type is realized as products such as, for example, compact disks having musical information stored therein and laser disks having image information stored therein. The write-once type is realized as products such as, for example, document files and still picture files. Currently, research and development is mainly performed on rewritable type. The rewritable type is being realized as products such as, for example, data files for personal computers.
A general optical disk has a transparent resin substrate having a thickness of 1.2 mm, a recording layer provided on one surface of the substrate, and a protective layer such as, for example, an overcoat provided on the recording layer. Another general optical disk includes a substrate and a protective plate formed of the same material as the substrate which are assembled together with an adhesive.
In order to increase the recording density of optical disks, studies have been recently performed for shortening the wavelength of the laser light and using an objective lens having a large numerical aperture (NA). However, as the wavelength of the laser light is shortened and the numerical aperture is increased, the tolerance for the angle of the disk with respect to the incident angle of the laser light (referred to as the xe2x80x9ctiltxe2x80x9d) is reduced. Reducing the substrate thickness (distance from the surface of the substrate to the recording layer) is effective in increasing the tolerance for the tilt. For example, the substrate thickness of a digital video disk (DVD) is 0.6 mm. Since the resin substrate having a thickness of 0.6 mm is insufficient in mechanical strength, two such substrates are assembled with the recording layer interposed therebetween.
The substrates are assembled by various methods, such as, for example, applying a hot melt resin on a surface of one substrate and then putting the substrates into contact with each other, assembling the two substrates with an adhesive tape (two-sided tape) interposed therebetween, or applying a ultraviolet (UV) curable resin on a surface of one substrate, then putting the two substrates into contact with each other and curing the resin with ultraviolet rays.
It has been found that a substrate having a thickness of as small as 0.6 mm is significantly warped when provided with a thin layer including a rewritable recording layer on a surface of the substrate.
Such a phenomenon does not occur when a substrate having a thickness of 0.6 mm is provided with a thin layer including a read-only recording layer, i.e., a metal reflective layer (formed of, for example, Al or Au) on a surface of the substrate. The phenomenon does not occur with a substrate having a thickness of 1.2 mm regardless of a thin film, including a rewritable recording layer or a read-only recording layer is provided.
The cause of the warp is considered to be as follows. In the case of the rewritable type disks, a dielectric layer is included in the thin layer in addition to the recording layer for protecting the recording layer. Formation of the dielectric layer generates a large stress, and the stress warps the substrate which has an insufficient mechanical strength.
When such a significantly warped substrate and another substrate which is not warped are assembled together to product an optical disk capable of one-side recording and reproduction, the resultant optical disk is also warped. Such a warped optical disk is not usable.
When two substrates which have the same thin layers and thus are warped at the same degree are assembled, a highly flat optical disk is obtained because the stresses generated in both substrates are balanced. However, use of two substrates both having the recording layers for producing an optical disk for one-side recording disadvantageously raises the production costs.
According to one aspect of the invention, an optical information recording medium includes a first substrate; at least a first dielectric layer and a recording layer for signal recording provided on a surface of the first substrate; and a second substrate. The first substrate and the second substrate are assembled together in the state of being warped in planar symmetry with respect to each other and flattened, the first dielectric layer and the recording layer being interposed between the first substrate and the second substrate.
In one embodiment of the invention, an optical information recording medium further includes a thin layer provided on a surface of the second substrate, the surface being opposed to the first substrate.
In one embodiment of the invention, the thin layer includes a second dielectric layer.
In one embodiment of the invention, the thin layer includes a second dielectric layer formed of an identical material as that of the first dielectric layer provided on the first substrate.
In one embodiment of the invention, the thin layer includes a metal layer.
In one embodiment of the invention, the thin layer has such a thickness to warp the second substrate to substantially an equal degree with the first substrate.
In one embodiment of the invention, an optical information recording medium further includes a resin layer formed of a resin provided on a surface of the second substrate which shrinks when being cured, the resin layer being provided on the surface not facing the first substrate.
In one embodiment of the invention, the resin layer is formed of a UV curable resin.
In one embodiment of the invention, the resin layer is transparent with respect to UV light.
In one embodiment of the invention, the resin layer is non-transparent with respect to visible light.
In one embodiment of the invention, the resin layer has graphics thereon.
In one embodiment of the invention, the second substrate is warped while being formed.
In one embodiment of the invention, the second substrate is formed by an injection method.
In one embodiment of the invention, at least one of the first substrate and the second substrate has a thickness of about 0.8 mm or less.
According to one aspect of the invention, an optical information recording medium includes a first substrate; at least a first dielectric layer and a recording layer for signal recording provided on a surface of the first substrate; a second substrate; and a thin layer on which a signal is unrecordable, the thin layer being provided on a surface of the second substrate. The first substrate and the second substrate are assembled together with the first dielectric layer, the recording layer and the thin layer being interposed therebetween.
In one embodiment of the invention, the thin layer includes a second dielectric layer.
In one embodiment of the invention, the thin layer includes a second dielectric layer formed of an identical material as that of the first dielectric layer provided on the first substrate.
In one embodiment of the invention, the first dielectric layer and the second dielectric have a substantially equal thickness.
According to one aspect of the invention, an optical information recording medium includes a first substrate; a plurality of first dielectric layers, a recording layer for signal recording provided between the plurality of dielectric layers, and a metal layer, which are provided on a surface of the first substrate; a second substrate; and a thin layer, including at least one second dielectric layer and a metal layer, on which a signal is unrecordable, the thin layer being provided on a surface of the second substrate. The first substrate and the second substrate are assembled together with the plurality of first dielectric layers, the at least one second dielectric layer, the recording layer, the metal layer, and the thin metal layer being interposed therebetween.
In one embodiment of the invention, the plurality of first dielectric layers and the at least one second dielectric layer has a substantially equal total thickness.
According to one aspect of the invention, an optical information recording medium includes a first substrate; at least a dielectric layer and a recording layer for signal recording provided on a surface of the first substrate; a second substrate; and a resin layer formed of a resin which shrinks when being cured, the resin layer being provided on a surface of the second substrate. The first substrate and the second substrate are assembled together with the dielectric layer and the recording layer being interposed therebetween, and the resin layer is not interposed therebetween.
In one embodiment of the invention, the resin layer is formed of a UV curable resin.
According to one aspect of the invention, an optical information recording medium includes a first substrate; at least a dielectric layer, a recording layer for signal recording provided on a surface of the first substrate, and a first resin layer formed of a resin which shrinks when being cured; and a second substrate. The first substrate and the second substrate are assembled together with the dielectric layer, the recording layer and the first resin layer being interposed therebetween
In one embodiment of the invention, an optical information recording medium further includes a second resin layer having a smaller tensile strength than that of the first resin layer, the second resin layer being provided on a surface of the second substrate, the second resin layer being provided on the surface facing the first substrate.
In one embodiment of the invention, the second resin layer has a smaller thickness than that of the first resin layer.
In one embodiment of the invention, the second resin layer is transparent with respect to UV light.
In one embodiment of the invention, the second resin layer is non-transparent with respect to visible light.
In one embodiment of the invention, the second resin layer has graphics thereon.
In one embodiment of the invention, at least one of the first substrate and the second substrate has a thickness of about 0.8 mm or less.
According to one aspect of the invention, a method for producing an optical information recording medium includes the steps of forming at least a first dielectric layer and a recording layer for signal recording on a surface of a first substrate; warping a second substrate; and assembling the first substrate, which is warped, and the second substrate in planar symmetry with respect to each other, and flattening an assembly of the first substrate and the second substrate.
In one embodiment of the invention, the step of warping the second substrate includes the step of forming a thin layer on a surface of the second substrate, the surface being opposed to the first substrate.
In one embodiment of the invention, the step of forming the thin layer includes the step of forming a second dielectric layer.
In one embodiment of the invention, the step of forming the thin layer includes the step of forming a second dielectric layer formed of an identical material with that of the first dielectric layer.
In one embodiment of the invention, the step of forming the thin layer includes the step of forming a metal layer.
In one embodiment of the invention, the step of warping the second substrate includes the step of forming a resin layer of a resin which shrinks when being cured on a surface of the second substrate, the resin layer being formed on the surface not facing the first substrate.
In one embodiment of the invention, the step of forming the resin layer includes the step of forming a UV curable resin layer.
In one embodiment of the invention, the step of warping the second substrate includes the step of causing a warp to the second substrate while the second substrate is being formed by an injection method.
In one embodiment of the invention, the second substrate is formed by an injection method.
According to one aspect of the invention, a method for producing an optical information recording medium includes the steps of forming at least a dielectric layer and a recording layer for signal recording on a surface of a first substrate; forming a resin layer of a resin which shrinks when being cured on the recording layer; and assembling the first substrate and a second substrate opposed to the first substrate with the dielectric layer and the recording layer being interposed therebetween.
Thus, the invention described herein makes possible the advantages of providing a flat and low-cost optical information recording medium, for one-side recording and reproduction, including two thin substrates assembled together, and a method for producing the same.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.